Metro Transit Police have arrested six people in an elaborate fare card scam that has so far netted the agency $16,000 worth of stolen Farecards, officials said yesterday. The investigation is ongoing, and officials do not know how much the counterfeit operation has cost the agency.

"This was a sophisticated operation to defraud a public agency," Metro General Manager John B. Catoe Jr. said at a news conference. "We think there is a Mr. Big, and that's who we would like to find."

The thieves traded in counterfeit paper Farecards in Metro Farecard machines to receive legitimate ones, or used the counterfeit ones to add value to electronic SmarTrip cards, officials said.

The thieves also sold some of the legitimate cards on the street at half-price, officials said. Metro is investigating whether the cards were also sold online. Because many transit agencies have similar fare collection systems -- a magnetic strip that is electronically read by a Farecard machine -- Metro has also alerted the American Public Transportation Association, a major industry group, so other agencies can be on the alert.

Metro put measures in place yesterday that will inconvenience riders but lessen the system's vulnerability to continued counterfeiting. Officials posted signs in stations and on fare card machines alerting them to the changes and urged riders to buy paper Farecards and SmarTrip cards directly from Metro vending machines in stations, sales centers or authorized Metro vendors.

The fraud was discovered this week but began early this year, officials said. Officials said the individuals created counterfeit paper Farecards that were detectable to a person but not to Farecard machines. The officials declined to provide details about how the operation worked or how the scam was detected. Officials have praised Transit Police, technology and financial transactions departments personnel for moving quickly to halt the theft.

According to a knowledgeable source who did not want to be identified because he was not authorized to speak, the scam worked like this: Thieves took a legitimate paper Farecard with $40 in value, sliced the card's magnetic strip into four lengthwise pieces, and then reattached one piece each to four separate defunct paper Farecards. The thieves then took the doctored Farecards to a Farecard machine and added fare, typically a nickel. By doing so, the doctored Farecard would go into the machine and a legitimate Farecard with the new value, $40.05, would come out.

The theft was detected when a Metro employee who monitors financial transactions noticed a large number of $40 Farecards being purchased at one time, the source said.

Officials say they do not know the magnitude of the operation. In the early 1990s, the agency lost about $300,000 from Farecard machines when thieves photocopied $5 bills and used them to get change or Farecards. In 2004, Metro's auditor estimated that $500,000 to $1 million in parking revenue was being lost to theft each year dating back to 2000.

Police have recovered $16,000 in Farecards stolen as part of the counterfeiting, along with 300 counterfeit cards.

In response to the fraud, Metro reprogrammed its fare machines yesterday to significantly lower the amount that passengers can trade in on used Farecards. The maximum, which used to be $40, is now $7, Metro spokeswoman Lisa Farbstein said.

At the same time, Metro is "turning off" all SmarTrip cards associated with the theft. Officials do not know how many cards will be affected. If the thieves have sold the counterfeit cards on the street and a person tries to add that value to a legitimate SmarTrip card, she said, Metro personnel will be able to track it and electronically disable the SmarTrip card.

Metro is also asking local governments to pass laws that make it illegal for unauthorized vendors to sell Farecards.

"We want anyone who tries to rob us to know that we're watching, and we will pursue you, arrest you and help prosecute you," said Metro Transit Police Chief Michael Taborn.